Pages

Monday, July 1, 2013

Defining greatness is subjective. Different people value and emphasize different things. There have been some defensive squads in the NFL whose greatness was undeniable. For those titans of the gridiron, the debate becomes which were better, and how do defenses of one era compare to those of another? This series will attempt to shed light on those questions while also exploring how far this young and talented Seahawks defense is from joining the ranks of the truly elite.

The rise has been steady throughout the past three decades. There was a dip in the 70s, where offenses struggled overall, but teams got much better at completing passes and reducing interceptions from the 80s forward. Defenses have been playing catch-up ever since.

Some teams have taken it to the extreme by assembling defensive talent and schemes that are somewhat vulnerable to the run in order to get sufficient pass rush talent on the field. The 2011 Super Bowl Champion New York Giants, for example, were 6th in interceptions and 3rd in sacks while only ranking 19th in rushing yards against and 23rd in opponent yards per carry. This article will attempt to shed more light on which were the best pass defenses to play in the NFL, and what made them stand out.

Note: Due to a variety of factors, the research for this series will only go back to 1960. If time allows, I will update with data from the 30s, 40s and 50s as well.

Relative Passer Rating

There may be no greater difference between the different eras of football than the passing game. That makes it all the more important to apply some relative weighting to the results of various pass defenses. Passer rating is an imperfect stat that does not take into account sacks or situation, but it is decent gauge of passing efficiency overall. Be sure to check out a great article by Kerry Byrne about the importance of opponent passer rating.

Sorting all defenses by opponent passer rating would be a flawed starting point since passer ratings were so much lower earlier in the NFL than they are now. Instead, I took the top three defenses in opponent passer rating from each NFL season dating back to 1960, and loaded them in a single spreadsheet. Then, I added in the average passer rating in the NFL for that season to help understand how those defenses were performing relative to the league at the time. Finally, I calculated the standard deviation for defensive passer rating in each year to see just how varied the teams were. An extremely large standard deviation implies the defenses were all over the map in defending the pass. A smaller standard deviation meant more defenses were performing at about the same level. The more standard deviations a defense was from the league average, the better their performance was relative to the year they played. The results are captured in the table below.

Warren Sapp, Ronde Barber, and Derrick Brooks helped the 2002 Bucs to the top of the charts. Marvel, for a moment, at their 48.4 opponent passer rating in the modern era of passing offenses. Only one team after 1980 held opposing passers to a lower rating, and that was the second place 1988 Minnesota Vikings.

It may not be an accident that those two teams finished 1-2 in the rankings. The Buc defense was coached by Monte Kiffin, while the Vikings secondary was coached by a Kiffin protege in Pete Carroll. Carroll also was the defensive coordinator for the 1995 49ers that are the 30th place team on the list. Seahawks fan excited about their young and talented secondary growing under the tutelage of Carroll can take that as a very promising sign. Those were the only two defenses to be a full three standard deviations below the league average in their time. It drops off significantly when going to the third place team, which is just over two standard deviations from the norm.

Only eight of the top 30 listed played before 1980, and none of the top 10. That comes as little surprise as it took an even more dominant pass defense to stand out when the league passing offenses still had training wheels on. The 1973 Steelers had the lowest opponent passer rating of this bunch at 33.1, but their accomplishment loses a little luster when you realize the league average was only 61.7 that year.

The 2009 New York Jets manage to place 10th here despite having the fewest interceptions of the group. That was Darrelle Revis' career year, and he helped to hold opposing quarterbacks to a minuscule eight passing touchdowns all year and a tiny 5.4 yards per attempt.

Chicago manages to get two seasons in the top 10 for both 1985 and 1986. They did it with sub-50% completion percentages and a bundle of interceptions (30+ in both years).

Pass defense is about more than rating, as mentioned earlier. Pass rush is not accounted for. Time to see which teams were the best at dropping the quarterback.

Sack packs

Many advanced stat keepers are quantifying pass pressure beyond the famous sack. Pressures, knockdowns and more can be found for recent years, but there is no record of those stats very far back. Even sacks did not officially show up as a stat until 1982. That is less than ideal for our purposes, but it is the best we have to go by. Pro-Football-Reference does include sack totals for teams dating back to 1960, so I will include those, but take them with a grain of salt.

A fair hypothesis would be that the more passing NFL offenses do, the more sacks defenses register. Not true. There was a massive spike in the mid-80s that was fueled both by special pass rush talent like Lawrence Taylor and an increase in pass attempts per game from an average of 25 in the mid-70s to over 32 by the mid-80s. Still, sacks as a percentage of pass attempts was significantly higher than it is today.

There is a clear downward trend in the ability of NFL defenses to sack the quarterback, at least as a percentage of pass attempts. Looking at just absolute sack numbers does not make a ton of sense since that does not account for variations in things like pass attempts. Instead, looking at sacks as a percentage of pass attempts should be more valuable. And, as with the other numbers we have looked at, we will factor in the league average sack rate for that season to see how outstanding the team was that posted the big sack number. To ensure the purity of these numbers, I only sampled the top sack teams from each season dating back to 1983--the first full season where sacks were tracked.

Chris Doleman, with his 21.0 sacks, combined with Keith Millard's 18.0 sacks to produce the most devastating pass rush defense of the group in 1989. The 71.0 total team sacks was bested by the 1984 Bears team that finished with 72.0, to go along with an eye-popping 14.2% sack rate. The league average back then was over a percentage point higher at 8.4%. That means the average sack rate for a team in 1984 would have been good for the 3rd-best pass rush in 2012. Context is king. The best pass rush team in 2012, the St. Louis Rams, place quite well in this list at 17th overall. Their 8.7% sack rate is barely over the average in 1984, but well over the 6.2% rate the league averaged last season.

It was surprising to see three New Orleans Saints teams in the top 10. Their duo of La'Roi Glover and Joe Johnson combined for 29.0 sacks in 2000 and another 17.0 in 2001. That 2000 squad featured another double digit sack producer in a young Darren Howard. They produced 40.0 sacks between the three of them that season.

The 2007 New York Giants included Osi Umenyiora, Michael Strahan and Justin Tuck to combine for 32.0 sacks and a Super Bowl upset of the undefeated Patriots. The late 80s, early 90s Philadelphia Eagles show up with the likes of Reggie White, Clyde Simmons and Jerome Brown. As do the 1990 Kansas City Chiefs with a young Derrick Thomas putting up 20.0 sacks all by himself.

Looking for an intersection

It would stand to reason that some of the best teams at holding opponent quarterbacks to low passer ratings would also be among the best pass rushing teams of their time. Yet, only one team--the 1991 Philadelphia Eagles--shows up on the best opponent passer rating list (#7) and the best pass rush teams (#16). Much of that is due to teams before 1983 being excluded from the sack rate list. Still, having a fearsome pass defense does not appear to be as dependent on having an elite pass rush as we have come to believe.

Championships

Another indication that pass rush is not as important a defensive trait as overall pass defense is the track record of the top teams for each of the tables above. Only one of the top 30 teams in adjusted sack rate made it to the Super Bowl--the 2007 Giants. Three of the top 10 is opponent passer rating won the Super Bowl, and six of the top 30 made it there, with five of them coming away with the ring.

The 2012 Seattle Seahawks

Carroll's Seahawks were third in the NFL in opponent passer rating last season at 71.8. That was good for 44th place on the adjusted list at -1.52 standard deviations from the league average. They were equivalent to the 2006 New England Patriots (-1.51), the 1993 Houston Oilers (-1.52) and the 1991 Washington Redskins (-1.53). Each of those teams won their division, and that Redskins team won the Super Bowl. The Patriots lost in the AFC Championship to the Colts. Seattle's pass defense is undoubtedly at a championship level, but they will need to do a much better job of intercepting passes and forcing more incompletions to join the ranks of historic pass defenses. Their 18 interceptions in 2012 would have been the second-worst of the top 30 teams in the list, and their 58% opponent completion rate would have been the worst of the bunch.

A better pass rush could be the key. Their 6% sack rate was actually below the league average of 6.2%. This research makes it clear that having the highest sack rate is not required to field a great pass defense, but a modest improvement above league average could vault their other pass defense numbers, especially interceptions and completion percentage.

The Best Pass Defense In History

Remarkably, the Vikings franchise gets their second straight nod for best of all-time, albeit a different era of Vikings football. The 1988 and 1989 Vikings finished 2nd and 1st respectively in adjusted opponent passer rating and adjusted sack rate. Many of the same players like Doleman, Millard, Henry Thomas and SS Joey Browner played on both teams. The 1988 team led the league in opponent touchdowns as well as in interceptions and opponent yards per attempt. They were second in opponent passing yards. They allowed only 12 passing touchdowns while picking off 36 passes. Opposing quarterbacks failed to even reach a 46% completion percentage over the course of the season. The two teams combined for only one playoff win, proving that it takes more than a dominant pass defense to win beyond the regular season.